1. Field of the Invention
The present invention relates to a photodetector utilizing plasmon resonance.
2. Related Background Art
Known as photodetectors utilizing light absorption of quantum intersubband transitions are QWIP (quantum well type infrared optical sensor), QCD (quantum cascade type optical sensor), QDIP (quantum dot infrared optical sensor), and the like. They utilize no energy bandgap transitions and thus have such merits as high degree of freedom in designing wavelength ranges and capability of high-speed response.
Among these photodetectors, the QWIP and QCD are equipped with a semiconductor multilayer body having a periodic multilayer structure such as a quantum well structure or quantum cascade structure. This semiconductor multilayer body generates a current due to an electric field component in the stacking direction thereof only when light incident thereon has such an electric field component, thus being not photosensitive to light having no electric field component in the stacking direction (planar waves incident thereon in the stacking direction thereof).
Therefore, in order for the QWIP or QCD to detect light, it is necessary for the light to be incident thereon such that a direction of vibration of an electric field of the light coincides with the stacking direction of the semiconductor multilayer body. When detecting a planar wave having a wavefront perpendicular to an advancing direction of light, for example, it is necessary for the light to be incident on the semiconductor multilayer body in a direction perpendicular to its stacking direction, which makes the photodetector cumbersome to use.
There has hence been known a photodetector in which, for detecting light having no electric field component in the stacking direction of the semiconductor multilayer body, a thin gold film is disposed on a surface of the semiconductor multilayer body and periodically formed with holes each having a diameter not greater than the wavelength of the light (see W. Wu, et al., “Plasmonic enhanced quantum well infrared photodetector with high detectivity”, Appl. Phys. Lett., 96, 161107 (2010)). In this example, the light is modulated so as to attain an electric field component in the stacking direction of the semiconductor multilayer body under a surface plasmonic resonance effect on the thin gold film.
Concerning the plasmon resonance, it has also been known that, when light is incident on a so-called MIM structure in which an insulator is held between metals, the electric field is remarkably enhanced by the plasmon resonance (see H. T. Miyazaki, Y. Kurokawa, “Squeezing Visible Light Waves into a 3-nm-Thick and 55-nm-Long Plasmon Cavity”, Phys. Rev. Lett., 96, 097401 (2006)).
On the other hand, the QDIP has three-dimensional directions for light confinement and thus can eliminate polarization dependence, thereby being able to detect light independently of the vibration direction of the electric field of light. However, the QDIP has a drawback in that it has intrinsically low photosensitivity due to its structure.